Conventional endoscopes have a large field of view that is in the range of about 90° to 140° so that tissues inside a body can be observed without overlooking lesions. They also change the distance to the object in order to obtain either magnified or reduced-sized images of an object to be observed, and thus have a large depth of field for a fixed focus point so that objects at distances between 3 mm and 50 mm can be observed without refocusing.
Conventional endoscopes also have an image scale factor with an absolute value of about 30 to 50 when the image is displayed on a monitor having a 14-inch screen, which is sufficient to observe diseased tissues. Zoom optical systems are used in order to obtain further magnified images, with the absolute value of the image scale factor being approximately 70 when displayed oil a monitor having a 14-inch screen. The zoom optical system typically has a built-in, zoom lens driving mechanism. As a result, the endoscope has an insert tip with an outer diameter that is larger than 10 mm and requires complex operations. Such endoscopes have limited applications.
The manner in which living tissues are observed using a conventional endoscope will now be described with reference to FIG. 1. Living tissues to be observed by a conventional endoscope often include a mucous membrane 1, transparent epithelial cells 2 and underlying parenchymal tissues 3 in which blood vessels run. Light illumination emitted at the endoscope tip part 4 must first pass through the mucous membrane and the transparent epithelial cells before reaching the parenchymal tissues. The illumination light which reaches the parenchymal tissues 3 is scattered by the parenchymal tissues 3. Of the light that is scattered by the parenchymal tissues, most re-enters the epithelial cells. The illumination light is also scattered by cell walls 5 and cell nuclei 6 when it is transmitted through the transparent epithelial cells. The light rays B1, B2 that are scattered by the cell nuclei of the epithelial cells are weak and thus the light rays that are scattered by the parenchymal tissues dominate. Consequently, in a conventional endoscope, only the parenchymal tissues are observed through an objective optical system.
When it becomes difficult to provide a diagnosis of an abnormality by observing images of a tissue, such as when a lesion is very small, a suspicious-looking tissue may be excised during the course of an endoscopic examination. The cells of the excised tissue are then examined under a microscope. Whereas an endoscope generally uses incident illumination from an illumination optical system that is positioned around an objective optical system, a microscope instead generally uses an objective optical system and an illumination optical system that are positioned on opposite sides of a sample. The sample is normally pre-processed in order to make it more suitable for observation, such as by removing the parenchymal tissues by slicing the sample thin in order to reduce scattering and/or by staining the sample in order provide better contrast.
The manner in which a sample is observed using a microscope will now be described with reference to FIG. 2. A prepared sample is fixed onto a cover glass 7 and illuminated from below with light from an illumination system 8. Illumination light rays A1′, A2′ are diffracted by the cell walls and cell nuclei as they transmit through the sample 9. The diffracted light rays B1′, B2′ interfere with one another both constructively and destructively, producing interference fringes that provide visible contrast. Thus, one can observe the sample by using an objective optical system 10 placed above the sample.
Laser-scanning-type confocal endoscopes which have a resolution sufficient for cellular observation have been proposed that may be inserted within a living body. These typically use a confocal optical system having a pinhole for passing an Airy disk light pattern at a position that is conjugate to the image plane, and the confocal optical system thus acquires diffraction-limited information for each point of an object in the field of view. A laser beam directed from a light emitting optical system scans the object, and information obtained from the reflected light from the object for each point is combined so as to produce an image representing either a two-dimensional or a three-dimensional object. High resolution can thus be realized not only within the image plane, but also in the depth direction.
It takes from several days to several weeks to identify abnormal tissue using conventional procedures wherein living tissues are excised and examined in vitro. Moreover, a cellular sample that is isolated and fixed for observation is only a tiny part of a removed tissue. Thus, although a cellular sample provides information on cellular structures, it is incapable of providing important functional information, such as information concerning fluid circulation within cells. This is because the circumstances between in vitro and in vivo examination are completely different. Thus, there is a need for magnifying endoscopes that will provide real-time, in-vivo observation of intact living cells.
In order to form cellular images of a lesion within a living body, a small-sized image pickup unit is necessary that is provided with an objective optical system with an image scale factor having an absolute value that is nearly as high as that of a microscope and which provides high resolution. The objective optical system used in a conventional endoscope does not meet these requirements. As mentioned previously, in a conventional endoscope as shown in FIG. 1, the illumination light is diffracted by the cell walls and cell nuclei as it transmits through the epithelial cells. The diffracted light rays B1, B2 are weak and the light rays A1, A2 that are scattered by the parenchymal tissues are dominant. Consequently, using a convention endoscope, only data from the parenchymal tissues is imaged by the objective optical system.
Although a conventional objective optical system as used in microscopes is satisfactory as far as providing sufficient imaging performance, such an objective optical system is too large for easy insertion into a living body. Laser-scanning-type confocal endoscopes have a problem in that their scanning speeds are still too slow for real-time, in vivo observations. Thus, as described above, an image pickup unit that meets the requirements for in vivo cellular observation has not yet been realized.